Thai police fired several canisters of tear gas and arrested up to 132 people as thousands of anti-government protesters attempted to march on Government House.
Seventeen thousand police were on duty around the historic Royal Plaza in central Bangkok for the demonstration organised by the opposition Pitak, or Protect Siam group, which prompted fears that it was the prelude to a coup attempt.
Protect Siam's leader retired general Boonlert Kaewprasit told the rally that he would "topple" this "corrupted and cruel government".
The group is an umbrella alliance of conservative, pro-royalist opponents to Thai Prime Minister Yingluck Shinawatra's ruling Pheu Thai party.
Minor clashes started in the morning and continued throughout the afternoon as some of the estimated 20,000 protesters confronted lines of riot police armed with shields and batons who had blocked off the roads leading to Government House with concrete blocks and razor wire.
The violence flared after demonstrators removed the coils of wire in an effort to force their way past the police. "We used tear gas because protesters were blocking police and did not comply with the security measures we had put in place," police spokesman Piya Uthaya told a local TV station.
Around 40 people, including a number of police, received minor injuries in the clashes, while knives were confiscated from some protesters.
Ms Yingluck, whose brother – exiled former prime minister Thaksin Shinawatra – was overthrown in a military coup in 2006, had earlier invoked Thailand's Internal Security Act, a special law which gives the police wider powers of arrest, and enables them to seal off roads and impose curfews, in response to the coup rumours.
In 2010 Bangkok was the scene of deadly street battles between supporters of Mr Thaksin, known as red shirts, and their opponents. Thailand remains deeply divided between the two groups, with the Protect Siam speakers and demonstrators expressing vitriolic comments about Ms Yingluck, whose Pheu Thai party won a large majority in elections last year.
"We're here to show we are against this crook, corrupt government," said Karmonwan Tangchai, a 43-year-old teacher from Bangkok. "Yingluck has no ability to rule; her brother tells her what to do. She disgraced the country last week by flirting with President Obama when he was in Thailand."
Protect Siam's spokesperson Air Marshal Vachara Ritthakhani told theSunday Telegraph that he expected at least 100,000 supporters to attend the rally and claimed that the invoking of the ISA had prevented many from reaching it.
Yet, with far fewer demonstrators turning out, Ms Yingluck's government has been given a major boost ahead of a three-day no-confidence debate in parliament called by the opposition Democrat Party that starts on Sunday.
